•Fig. 25.4 Diagram of the Basic Wiring of the Brainstem Ventilatory Controller. Thesignsofthemainoutput(arrows) oftheneuronpoolsindicatewhethertheoutputisexcitatory(+)orinhibitory(−).PoolAprovidestonicinspiratorystimulitothemusclesofbreathing.PoolBisstimulatedbypoolAandprovidesadditionalstimulationtothemusclesofbreathing,andpoolBstimulatespoolC.OtherbraincentersfeedintopoolC(inspiratorycutoffswitch),whichsendsinhibitoryimpulsestopoolA.Afferentinformation(feedback)fromvarioussensorsactsatdifferentlocations:ChemoreceptorsactonpoolA,andintrapulmonarysensoryfibersactviathevagusnervesonpoolB.Apneumotaxiccenterintheanteriorponsreceivesinputfromthecerebralcortex,anditactsonpoolC.
from cells in these areas excite some cells and inhibit other cells.
At the level of the respiratory control center, inspiration and exhalation involve three phases: one inspiratory and two expiratory (
Fig. 25.4 ). Inspiration begins with an abrupt increase in discharge from cells in the nucleus tractus solitarius, the nucleus retroambiguus, and the nucleus para-ambiguus, followed by a steady ramp-like increase in firing rate throughout inspiration. This leads to progressive contraction of the respiratory muscles during automatic breathing. At the end of inspiration, an “off-switch” event causes neuron firing to decrease markedly, at which point exhalation begins. At the start of exhalation (phase I of expiration), a paradoxical increase in inspiratory neuron firing slows the expiratory phase down by increasing inspiratory muscle tone and expiratory neuron firing. This inspiratory neuron firing decreases and stops during phase II of exhalation. Although many different neurons in the DRG and VRG are involved in ventilation, each cell type appears to have a specific function. For example, the Hering-Breuer reflex is an inspiratory-inhibitory reflex that arises from afferent stretch receptors located in the smooth muscles of the airways. Increasing lung inflation stimulates these stretch receptors and results in early exhalation by stimulating the neurons associated with the off-switch phase of inspiratory muscle control. Thus rhythmic breathing depends on a continuous (tonic) inspiratory drive from the DRG and an intermittent (phasic) expiratory drive from the cerebrum, thalamus, cranial nerves, and ascending sensory tracts in the spinal cord.
A chemoreceptor is a receptor that responds to a change in the chemical composition of blood or other fluid around it. Central chemoreceptors are specialized cells on the ventrolateral surface of the medulla. Chemoreceptors are sensitive to the pH of the surrounding extracellular fluid. Because this extracellular fluid is in contact with cerebrospinal fluid (CSF), changes in the pH of CSF affect ventilation by acting on these chemoreceptors
CSF is an ultrafiltrate of plasma that is secreted continuously by the choroid plexus and is reabsorbed by the arachnoid villi. Because it is in contact with the extracellular fluid in the brain, the composition of CSF is influenced by the metabolic activity of the cells in the surrounding area and the composition of the blood. Although the origin of CSF is plasma, the composition of CSF is not the same as that of plasma because the blood-brain barrier exists between the two sites (